Cathode-ray tube



P 1954 P. J. H. JANSSEN 2,689,923

CATHODE-RAY TUBE Filed Oct. 12. 1951 DEFLECTION Cows l- MAGME r: c 8 LAyEQs INVENTOR Peter Johannes Huberfus Jan sen Agent Patented Sept. 21, 1954 CATHODE-RAY TUBE Peter Johannes Hubertus Janssen, Eindhoven, Netherlands, assignor to Hartford National pany, Hartford, Conn., as

Bank and Trustv Com trustee Application October 12, 1951, Serial No. 250,983

Claims priority, application Netherlands November 1, 1950 2 Claims. (01. 313-46) :1 This invention relates to cathode-ray tubes, having a coil for deflecting the electron beam, wherein the radius of curvature of the surface scanned by the beam exceeds the distance of this surface from the centre of the active length of the deflection coil.

Such a combination is, for example, employed in modern television receivers, in which the surface of the cathode-ray tube to be scanned is substantially flat and of such a size that the beam must be deflected over a large angle both for horizontal and vertical scanning.

If this surface has a spherical shape and the centre of the sphere substantially coincides with the centre of the effective length of the deflection coil i. e. with the point at which the deflection action of the coil may be imagined to be concentrated, the displacement of the point of scanning spot over thesurface is proportional to the intensity of the deflecting magnetic field.

If, however, cathode-ray tubes having a substantially flat scanning surface are employed in combination with normal deflection coils producing a substantially homogeneous magnetic field, a raster having a pronounced cushion-like distortion and a poor linearity is obtained in the case of strong deflection, for example, in horizontal and vertical scanning in a television receiver.

It is difiicult to obviate this distortion by altering the shape of the saw-tooth deflection currents of the coils, since otherwise the line sawtooth current similarly to the frame saw-tooth current, must be modulated in the rhythm of line and frame.

The present invention has for its object to obviate the aforesaid drawback in a simple manner has the feature that the deflection coil comprises a shell consisting of at least two separated layers of ferromagnetic material surrounding the coil substantially throughout its active length.

In order that the invention may be readily carried into effect, an example will now be described in detail with reference to the accompanying drawing.

The single accompanying figure is a crosssection of a coil system, according to one embodiment of the invention, in a plane at right angles to the longitudinal axis of the system and, at the centre of the active length.

The neck I of the cathode-ray tube is surrounded by a line-deflection coil consisting of coil halves 2 and 3, and a frame-deflection coil consisting of coil halves 4 and 5.

The coils may be of normal construction so that the individual turns (not shown) extend at right angles to the plane of the drawing and substantially parallel to the tube wall I also extending at right angles to the plane of the drawing. The two deflection systems are surrounded by a shell consisting in this case of three layers E, I and 8 of ferromagnetic material, between which insulating layers 9 and ID are sandwiched.

The layers 6, 1 and 8 extend at right angles to the plane of the drawing substantially throughout the active length of the coils and surround them.

Each layer may consist of band-shaped material, the band being wound on the coil bases in such manner that the winding pitch corresponds to or slightly exceeds the width of the band so that, viewed in a longitudinal direction of the coil, the windings just do not overlap and, notwithstanding that, form a practically continuous surface so that the layers may be operative as magnetic short-circuit turns, with the result that the magnetic field is concentrated within these rings.

With an increase in strength of the deflection currents through the deflection coils, the magnetic flux successively will saturate the layer 6, the layer 1 and the layer 8 which involves a slower increase in strength of the magnetic deflection field than corresponds to the occurring amplitude of the deflection current, so that the beam of the cathode-ray tube is less deflected.

Since the saturation is caused both by the line-deflection and by the frame-deflection currents, the required intermodulation occurs.

By a judicious choice of the thickness of the ferromagnetic layers and their position, which choice is determined by the value of the angle of deflection and the ratio of the aforesaid radii of curvature, a very satisfactory compensation of raster disfigurement and non-linearity is ensured.

For example, in a practical case where the total angle of deflection of the beam was ten layers consisting of mu metal having a thickness of 50 mu were used, which layers were separated by paper layers of mu thick.

What I claim is:

1. A deflection coil system for a cathode ray tube in which the radius of curvature of a surface to be scanned exceeds the distance of this surface from the center of the active length of the coil system comprising a deflection coil and a ferromagnetic shell surrounding said coil, said 4 2. A deflection coil system as claimed in claim 1 in which the layer of insulating material is paper.

5 References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 0 2,155,514 Tolson et a1 Apr. 25, 1939 2,167,379 Tolson July 25, 1939 2,207,777 Blaln July 16, 1940 

